Electrolyte media for the deposition of tin alloys and methods for depositing tin alloys

ABSTRACT

An electroplating bath medium for electroplating articles with a tin-cobalt, tin-nickel, or tin-cobalt-nickel alloy comprises: at least one tin salt; an alloying metal salt comprising a cobalt salt and/or a nickel salt; a complexant comprising a hydroxycarboxylic acid or alkali metal salt thereof such as a sodium or potassium gluconate or heptonate complexant; boric acid; and a bath soluble substituted phenolic compound. The current regime applied to the plating bath can include time intervals of direct current and of pulsed current in order to selectively control the deposition of tin by activation or diffusion control.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/GB02/01044, filed Mar. 13, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to the deposition of tin alloys,especially tin-cobalt alloys, tin-nickel alloys and tin-nickel-cobaltalloys and to electrolyte media for use in the deposition of suchalloys. The invention relates especially to the use of such alloys asbearing overlays.

The “End of Life” vehicle regulations in Europe for 2003 aim to increasethe “recyclability” of vehicles by removing toxic materials such ashexavalent chromium and lead. One common use of lead in automotiveapplications is the use of lead alloys in bearing overlays. Overlays aregenerally soft alloys deposited onto harder bearing alloys to produce asurface having compatibility and conformability with a co-operatingshaft and also to provide a means of embedding debris particles toprevent damage to the shaft. More than 300,000,000 bearing shells arecurrently plated every year. The most commonly applied bearing overlaymaterial is a lead-tin-copper alloy containing at least 90% lead. Inhigher performance engines, lead-indium (where the indium is plated ontop of the bearing and diffused into the underlying lead) is commonlyapplied. Clearly, in order to comply with the 2003 regulations, areplacement for lead must be found.

A suitable replacement alloy must be soft enough to allow the bearing to“bed in” correctly and the melting point of the alloy must be higherthan 250° C. because engine operating temperatures can approach thislevel. Tin-based alloys are an obvious choice as they have goodlubrication properties and are soft enough. The low toxicity of tin isalso an advantage. Tin cannot be used alone because the melting point oftin is too low. The easiest alloy of tin to produce would be atin-copper alloy. A tin-copper alloy containing approximately 5% copperwould have the required melting point. However, tests have shown thattin-copper alloys do not have the necessary fatigue strength. Tin zincalloys can readily be produced but these alloys fail corrosion testingdue to the appearance of white corrosion products from the sacrificialcorrosion of the zinc in the alloy.

Other tin based alloys include tin-nickel, tin-cobalt or ternary alloyscomprising all three of these metals. The use of these alloys forbearing overlays has already been suggested. In particular, tin cobalthas been found to be advantageous. U.S. Pat. No. 4,795,682 discloses theuse of tin cobalt alloys containing preferably between 2-8% cobalt.These alloys are claimed to have superior fatigue resistance as comparedto standard lead-tin-copper bearing overlays (90-100 Mpa as compared to60-70 Mpa when tested on a “Sapphire” testing machine). However, as faras the applicant is aware, these alloys have not been exploitedcommercially due to the difficulty of producing a tin cobalt alloy ofthe required composition and thickness. According to U.S. Pat. No.4,795,682, the tin cobalt alloy overlays were produced by the techniqueknown as “Brush Plating” where the coating is applied manually bybrushing the bearing with an anode coated with an absorbent materialsoaked in an electrolyte containing tin and cobalt salts and a gluconatecomplexant. This technique is not easily applicable to mass productiontechniques and so the use of tin-cobalt alloy for bearing overlays hasnot been possible in commercial practice in spite of its performanceadvantages.

The production of tin-cobalt alloys is also described in other prior artdocuments. One commercial use of tin-cobalt alloys is in the productionof thin overlays for nickel plated components as a replacement forchromium. However, it is not possible to produce thick coatings fromthese electrolytes as the content of tin is only 2-4 g/l. Theseelectrolytes also produce coatings of a composition approximating to theintermetallic alloy composition (20-25% cobalt), whereas the optimumdesired composition is about 2-8% cobalt for bearing alloys since theintermetallic composition is too hard.

Several compositions have been proposed in the patent literature whichclaim to be suitable for producing thicker deposits of tin-cobaltalloys.

U.S. Pat. Nos. 3,951,760 and 4,021,316 suggest an alkaline bath based onpyrophosphate and utilising an organo sulphur compound as a brighteningagent and peptones as grain refining agents. Baths based onpyrophosphate have the disadvantage that the stannous tin ions are notstable in alkaline media and quickly oxidise to stannic tin renderingthe bath useless. Also, insoluble anodes have to be used in these bathsas tin does not dissolve effectively in these pyrophosphate baths. Thesebaths would therefore be unsuitable for the high volume production ofplated bearings.

Several patents have been granted for baths based on stannous chlorideand cobalt chloride also containing fluoride in order to complex the tinions and facilitate co-deposition of cobalt (U.S. Pat. Nos. 3,966,564and 4,029,556). These electrolytes are very corrosive and toxic andbecause they contain large amounts of ammonium ions, they are difficultto effluent treat. Additionally, these electrolytes produce anintermetallic alloy over a wide range of current densities and so areunsuitable for producing alloys of the required composition.

U.S. Pat. No. 4,168,223 describes a citrate-based bath from which it isclaimed tin-cobalt alloys could be deposited However, attempts by thepresent applicant to reproduce the examples cited in U.S. Pat. No.4,168,223 resulted in only deposits of pure tin with no co-deposition ofcobalt (when examined by Energy Dispersive X-ray Analysis).

A more recent patent (U.S. Pat. No. 4,828,657) discloses baths based onstannic tin in either alkaline or acidic media. Maintenance of tinconcentration in these baths is very difficult as it is not possible todirectly dissolve tin anodes in stannic baths because build-up ofstannite ions in the bath leads to spongy deposition. Also, if acidbaths are formulated based on stannic tin, the tin tends to eventuallyprecipitate as alpha or metastannic acid.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method of depositing smooth,functional alloy coatings of tin and an alloying metal comprising nickeland/or cobalt, the coating having a thickness of up to and in excess ofabout 50 microns and a composition of about 1-25% (preferably about 2 to15% and especially about 2 to 8%) of the alloying metal and to provideelectrolyte media and electroplating baths and bath media suitable foruse in the method. The invention also relates to tin alloys and bearingoverlays produced by the method.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts a schematic representation of a portion of a directcurrent and pulsed current cycle in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to a first aspect of the invention, there is provided aprocess for electroplating a substrate with a tin alloy, which alloycomprises from about 2% by weight to about 15% by weight of alloyingmetal, said process comprising:

-   -   a) contacting the substrate with an electroplating bath medium        comprising:        -   1. a soluble stannous tin salt;        -   2. a soluble salt of the alloying metal; and        -   3. a complexing agent;    -   b) applying a current regime which includes a first condition in        which the average current density is such that the alloying        metal is deposited and a second condition in which the average        current density is lower than in the first condition such that        substantially only tin is deposited.

It is particularly preferred that the current regime comprises applyinga pulsed current, and especially that the current regime comprisesapplying a direct current for at least one first time interval andapplying a pulsed current for at least one second time interval.

Preferably the tin salt, the alloying metal salt and the complexingagent respectively are selected from those set out in accordance withthe second aspect of the invention below.

According to a second aspect of the present invention, a composition forthe preparation of an electroplating bath medium for electroplating oftin and an alloying metal comprising cobalt and/or nickel alloyscomprises:

-   -   (a) at least one tin salt selected from the group comprising        stannous sulphate, stannous chloride, stannous        methanesulphonate, stannous fluoborate, stannous        phenolsulphonate and stannous toluenesulphonate;    -   (b) at least one alloying metal salt selected from the group        comprising cobalt sulphate, nickel sulphate, cobalt chloride,        nickel chloride, cobalt methanesulphonate, nickel        methanesulphonate, cobalt fluoborate, nickel fluoborate, cobalt        phenolsulphonate, nickel phenolsulphonate, cobalt        toluenesulphonate and nickel toluenesulphonate;    -   (c) one or more bath complexants selected from the group        comprising mono-, di- or poly-hydroxy carboxylic acids or alkali        metal salts thereof; and    -   (d) boric acid.

The tin salt is preferably present in a concentration range of about 2to about 80 g/l. Either the cobalt salt or the nickel salt or acombination of both may be used. A concentration range of about 10 toabout 250 g/l in total of the salt(s) of group (b) will preferably bepresent in the plating bath. The bath complexing agent (complexant) willpreferably be present in the bath in a concentration of about 20 toabout 200 g/l. Although polycarboxylic acid are presently preferred asthe bath complexant, mono- and di-carboxylic acids respectively providegood results and are also favored in the present invention. Preferredexamples of suitable bath complexants include lactic acid, malic acid,tartaric acid, gluconic acid, glucoheptonic acid, ascorbic acid,glycolic acid, and citric acid or alkali metal salts thereof. The boricacid is preferably present in the bath in a concentration range of about10 to about 150 g/l.

According to a third aspect of the invention there is provided anelectroplating bath medium for electroplating tin-cobalt or tin-nickelalloys comprising the composition of the second aspect of the invention,and

-   -   e) a grain refining agent comprising a bath soluble substituted        phenolic compound of the formula:

where n is an integer from 1 to 100, m is 2 or 3 and R1 is either aphenolic group of the formula:

where R2 is a (C1 to C14) alkyl group or, an alpha or beta naphthalenegroup of the formula:

and, R3 is selected from the group comprising H, CH₂CH₂OH, CH₂CH₂CH₂OH,CH₂CH(OH)CH₂SO₃H and CH₂CH₂CH₂SO₃H.

These compounds are preferably present in the bath in concentrations ofbetween about 10 ppm and about 10 g/l.

In addition to these essential ingredients, the bath medium mayoptionally contain anionic wetting agents to reduce surface tension andaid the dissolution of the grain refining phenolic compounds (e) in thebath medium. Salts such as sodium sulphate and/or ammonium chloride maybe added to increase the conductivity of the bath medium.

A fourth aspect of the invention provides an electroplating bathcomprising an electroplating bath medium according to the second aspectof the invention and a vessel for containing the medium. Preferably, theelectroplating bath comprises at least one tin anode.

A fifth aspect of the present invention provides a process forelectroplating an article with a tin-alloy coating, which processincludes the step of immersing the article in an electroplating bathaccording to the third fourth of the invention.

In preferred embodiments of the fifth aspect of the invention, theprocess includes the step of utilising a current regime which includes afirst condition in which the average current density is such that the atleast one alloying metal is deposited and a second condition in whichthe average current density is lower than in the first condition suchthat substantially only tin is deposited. In the first condition, tinmay also be deposited with at least one alloying metal.

The invention also relates to electroplated articles produced by theprocesses of the invention and especially to electroplated bearingswherein the electroplated coating constitutes a bearing overlay.

Bath media formulated according to the invention are stable inoperation, contain no highly toxic or corrosive components and can bereadily effluent treated. The operating pH of the medium bath in anelectroplating bath is preferably between 2 and 3 (although the bath maybe operated between pH 1.5 to 6). At this pH, tin anodes may be utilisedso the electroplating process may be operated on a continuous basis.Without wishing to be bound by theory, the applicants believe that theboric acid reacts with the hydroxyl groups on the complexant species toform boro-hydroxycarboxylate complexes. Typically, the baths areoperated at temperatures from about 20 to 70° C. and at currentdensities (subject to the pulsed current techniques discussed below)between about 2 and about 5 A/dm².

Deposits produced from the baths of the present invention are smooth andfine-grained. By using direct current with the electrolyte medium of theinvention to plate Hull cell panels, and analysing the depositcomposition at various positions along the panel, the present inventorshave been able to determine a relationship between current density andalloy composition. The inventors have found that below a certainthreshold current density (typically between 2 and 3 A/dm²), a depositconsisting essentially of 100% tin (analysed by energy dispersive X-rayanalysis) is typically deposited. Above this current density, an alloyis deposited of composition dependent on the bath formulation,temperature and pH.

Thus, the inventors have appreciated that alloy coatings produced fromthe electrolyte medium of the invention show a particular compositionaldependence on the applied current density. Without wishing to be boundby theory, the inventors postulate that at low current densities thedeposition process is under activation control and a deposit consistingessentially of pure tin results. At higher current densities, thedeposition of tin is under diffusion control and this mass-transportlimitation of tin deposition allows co-deposition of cobalt or nickel. Avery large difference in the composition of the obtained alloy coatingresults as the electroplating moves (with increasing current density)from activation control to diffusion control. This relatively suddenchange conventionally makes fine control of the composition of thedeposited alloy difficult to achieve.

However, the inventors have appreciated that this difficulty can beturned to significant advantage. In accordance with the presentinvention, the current density during plating can be varied, or pulsedcurrent can be used, in order to control, in any given time period,whether the plating system is acting under diffusion control oractivation control of tin deposition. In this way, fine control of thedeposited alloy composition is achievable, or compositionally modulatedalloy layers can be deposited. Pulsed current techniques per se areknown in the art.

For example, a compositionally modulated alloy of the correct desiredaverage composition may be achieved by utilising a current which“surges”, i.e. alternates, between high and low current density.

Alternatively, the composition of the deposited alloy can be controlledby using a pulsed current. Preferably, a current regime is employedincluding time intervals of direct current and of pulsed current. In thedirect current time interval, deposition proceeds under diffusioncontrol. By switching between direct current and pulsed current,activation control of tin depositon is re-established in the pulsedcurrent cycle because the “off” time between current pulses allows timefor tin to diffuse to the cathode surface. Adjustment to the relativelengths of the direct current and pulsed current time intervals andadjusting also (or alternatively) the duty cycle (as defined below) ofthe pulsed current can be used to compensate for compositionalvariation, temperature variation or hydrodynamic variation in a platingbath, thereby ensuring consistent plating results. A schematicrepresentation of a portion of a direct current and pulsed current cycleis shown in FIG. 1.

Thus in FIG. 1, time interval I represents a period of direct currentand time interval II represents a period of pulsed current. In each timeinterval I, the current density is such that the diffusion control oftin deposition is achieved. In the time interval II as illustrated, thepulsed current changes between an “on” pulse period at which the currentdensity is the same as that in time interval I, and an “off” pulseperiod at which the current density is substantially zero. In variationsof the invention, in time interval II the current density in the “on”pulse period need not be the same as the current density in timeinterval I, provided only that the currently density is sufficient toenable diffusion control of tin deposition to be established. Also, inthe “off” pulse period of time interval II, the current density need notbe zero, provided that it is low enough to allow activation control oftin deposition, allowing time for tin ions to diffuse to the cathodesurface. The duty cycle, mentioned above, is defined as the percentageof the pulsed current time period (time interval II) when the current isin an “on” pulse period.

The invention will now be described with reference to working examples:

EXAMPLE 1

A bath medium was made to the following formulation:

Sodium Gluconate 150 g/l Boric Acid 100 g/l Stannous Sulphate Anhydrous30 g/l Cobalt Sulphate Hexahydrate 100 g/l Lugalvan BNO 24* 1 g/lEmpicol ESB3 Anionic Surfactant** 1 g/l *Lugalvan BNO 24 is anethoxylated beta-napthol in accordance with a compound of class e). Itis manufactured by BASF. **Empicol ESB3 is an anionic surfactantmanufactured by Albright & Wilson PLC.

The bath was heated to 50° C. and a Hull cell test was performed at 1amp for 10 minutes using agitation by mechanical stirrer. The resultantdeposit was smooth and even. At a position on the panel corresponding toa primary current density of 4 A/dm², a tin cobalt alloy was obtainedhaving a composition of approximately 16% cobalt with the balance beingtin. This example illustrates that a significant decrease in the amountof cobalt (as compared to the intermetallic composition) is achievedusing the electrolyte medium of the invention. A still further reductionin the amount of cobalt is, however, desirable.

EXAMPLE 2

A bath medium was made to the following formulation:

Sodium Gluconate 150 g/l Boric Acid 100 g/l Stannous Sulphate Anhydrous30 g/l Nickel Sulphate Hexahydrate 75 g/l Cobalt Sulphate Hexahydrate 7g/l Lugalvan BNO 24 1 g/l Empicol ESB3 1 g/l

A Hull cell test as in Example 1 was performed at a temperature of 20°C. The deposit obtained was smooth and even. At a position on the panelcorresponding to a primary current density of 4A/dm², a tin nickelcobalt alloy was obtained having a composition of 8-10% nickel and 1-2%cobalt with the balance being tin. This example illustrates that aternary tin-cobalt-nickel alloy can be produced using the electrolytemedium, resulting in an alloy having useful amounts of nickel andcobalt.

EXAMPLE 3

A Hull cell test was performed on a bath of composition as in Example 1,but substituting nickel sulphate hexahydrate for cobalt sulphatehexahydrate, and using the same conditions as in Example 1. A tin-nickelalloy was obtained having a composition of about 14% nickel, with thebalance being tin.

EXAMPLE 4

A Hull cell test was performed on a bath of composition as in Example 1at a temperature of 60° C. A tin cobalt alloy was obtained at 4 A/dm²having a composition of 21% cobalt with the remainder being tin.

EXAMPLE 5

A Hull cell test was performed on a bath of composition as Example 2 ata temperature of 60° C. A tin nickel cobalt alloy was obtained at 4A/dm² having a composition of 15% nickel, 3% cobalt and the remainderbeing tin.

EXAMPLE 6

2 liters of the bath medium of Example 1 was placed in a plating cellequipped with mechanical agitation and a tin anode. A small test piecewas plated at an average current density of 4 A/dm² at a bathtemperature of 60° C. The resulting deposit was analysed and found toconsist of 22% cobalt.

A test piece was then plated at 2 A/dm² in a similar manner. Theresulting deposit had a composition of 100% tin.

A third sample was produced using pulsed current A programmablepulse-plating unit (JCT Controls Ltd) was used with the followingparameters:

Channel 1 Direct Current 20 seconds Channel 2 Pulsed current 100 Hz, 30%duty cycle 60 seconds

The plating voltage was adjusted so that during the DC cycle (Channel1), the current density on the test piece was 4 A/dm². The sample wasplated for 15 minutes (the average current density was observed to fallapproximately 1.3 A/dm² during pulsed current cycle). The resultingdeposit was analysed and found to be 5.5% cobalt with the remainderbeing tin. This is the ideal composition for bearing overlays.

EXAMPLE 7

A sample was plated using the same method as the pulsed current sampleof Example 5 but using the following parameters:

Channel 1 Direct Current 30 seconds Channel 2 Pulsed Current 100 Hz 30%duty cycle 30 secondsThe alloy composition of a test piece plated with these parameters wasdetermined to be 10.2% cobalt. This example serves to illustrate theease of control of the deposit composition by adjusting pulseparameters.

EXAMPLE 8

The bath medium of Example 1 was used in the plating cell of Example 5to produce a test piece. A temperature of 60° C. was used. A test piecewas plated using alternate current densities of 4 and 2 A/dm² (30seconds each) to produce a layered coating. The deposit composition wasanalysed and found to be 10.5% cobalt. This example illustrates analternative means of controlling the alloy composition.

EXAMPLE 9

A bath medium was made to the following formulation;

Stannous sulphate anhydrous 30 g/l Cobalt sulphate hexahydrate 100 g/lBoric acid 100 g/l Sodium potassium tartrate tetrahydrate 150 g/lRalufon NAPE 14-90*** 2 g/l ***Ralufon NAPE 14-90 is a sulphopropylated2-naphthol ethoxylate available from Raschig GmbH.

The bath was adjusted to pH 3.0 with sulphuric acid or sodium hydroxidesolution as required, heated to 50° C. and a Hull cell test wasperformed under the following pulse plating regime:

Channel 1 Direct Current 20 seconds Channel 2 Pulsed current 100 Hz, 30%duty cycle, 60 seconds.

The current used was 1 amp during the d.c. cycle, this fell to approx0.3 amps (average) during the pulse cycle. The plating time was 20minutes. The panel was analysed and found to contain 3.5% cobalt at apoint corresponding to a current density of 4 A/dm² during the d.c.cycle.

This example illustrates the use of a di-hydroxy carboxylic acid.

EXAMPLE 10

A bath medium was made to the following formulation;

Stannous sulphate anhydrous 30 g/l Cobalt sulphate hexahydrate 100 g/lBoric acid 100 g/l Sodium potassium tartrate tetrahydrate 50 g/l Sodiumgluconate 100 g/l Ralufon NAPE 14-90*** 2 g/l

The bath was adjusted to pH 3.0, heated to 50° C. and a Hull cell testwas performed under the same conditions as Example 9.

The panel was analysed and found to contain 7.3% cobalt at a pointcorresponding to a current density of 4 A/dm² during the d.c. cycle, and3.2% cobalt at a point corresponding to 3 A/dm².

EXAMPLE 11

A bath medium was made to the following formulation;

Stannous sulphate anhydrous 30 g/l Cobalt sulphate hexahydrate 100 g/lBoric acid 100 g/l Sodium potassium tartrate tetrahydrate 100 g/l Sodiumgluconate 50 g/l Lugalvan BNO24 1 g/l Empicol ESB3 1 g/l

The bath was adjusted to pH 3.8, heated to 50° C. and a Hull cell testwas performed under the same conditions as Example 9.

The panel was analysed and found to contain 5.3% cobalt at a pointcorresponding to a current density of 3 A/dm² during the d.c. cycle.

EXAMPLE 12

A bath medium was made to the following formulation;

Stannous sulphate anhydrous 30 g/l Cobalt sulphate hexahydrate 100 g/lBoric acid 100 g/l Malic acid 100 g/l Lugalvan BNO24 1 g/l Empicol ESB31 g/l

The bath was adjusted to pH 3.8, heated to 50° C. and a Hull cell testwas performed under the same conditions as Example 9.

The panel was analysed and found to contain 3.4% cobalt at a pointcorresponding to a current density of 4 A/dm² during the d.c. cycle.

This example illustrates the use of a mono-hydroxy carboxylic acid.

1. A process for electroplating a substrate with a tin alloy, whichalloy comprises from about 2% by weight to about 15% by weight ofalloying metal, said process comprising: a) contacting the substratewith an electroplating bath comprising:
 1. a soluble stannous tin salt;2. a soluble salt of the alloying metal; and
 3. a complexing agent; b)applying a current regime in which a direct current having asubstantially constant current density is applied for a first timeinterval and a pulsed current is applied for a second time interval,wherein said first time interval and said second time interval areconsecutive, wherein the pulsed current comprises a first pulse periodat a first current density and a second pulse period in which thecurrent density is substantially zero, wherein the first pulse periodand the second pulse period are alternated, whereby a duty cycle ofpulsed current is established; and wherein during the first timeinterval the alloying metal is deposited along with the tin and duringthe second time interval substantially only tin is deposited.
 2. Aprocess as claimed in claim 1 wherein the alloying metal is selectedfrom the group consisting of cobalt, nickel and combinations of theforegoing.
 3. A process according to claim 1 wherein the electroplatingbath further comprises boric acid.
 4. A process according to claim 1wherein the tin salt is selected from the group consisting of stannoussulphate, stannous chloride, stannous methanesulphonate, stannousfluoborate, stannous phenolsulphonate and stannous toluenesulphonate. 5.A process according to claim 1 wherein the alloying metal salt isselected from the group consisting of cobalt sulphate, nickel sulphate,cobalt chloride, nickel chloride, cobalt methanesulphonate, nickelmethanesulphonate, cobalt fluoborate, nickel fluoborate, cobaltphenolsulphonate, nickel phenolsulphonate, cobalt toluenesulphonate andnickel toluenesulphonate.
 6. A process according to claim 1 wherein thecomplexing agent is selected from the group consisting of mono-, di- orpoly-hydroxy carboxylic acids or alkali metal salts thereof.
 7. Aprocess according to claim 1 wherein the electroplating bath furthercomprises a grain refining agent comprising a bath soluble substitutedphenolic compound of the formula:

where n is an integer from 1 to 100, m is 2 or 3 and R1 is either aphenolic group of the formula:

where R2 is a (C1-C14) alkyl group or, an alpha or beta naphthalenegroup of the formula:

and, R3 is H, CH₂CH₂OH, CH₂CH₂CH₂OH, CH₂CH(OH)CH₂SO₃H or CH₂CH₂CH₂SO₃H.8. A process as claimed in claim 1 wherein the first time interval isfrom about 10 seconds to about 100 seconds and second time interval isfrom about 20 seconds to about 200 seconds.
 9. A process as claimed inclaim 1 wherein the pulsed current has a frequency of from about 10 Hzto about 500 Hz.
 10. A process as claimed in claim 1 wherein said firstcurrent density is substantially the same as the current density in thefirst time interval.
 11. A process as claimed in claim 10 wherein theduty cycle of the pulsed current is from about 20% to about 50%.
 12. Aprocess as claimed in claim 1 wherein the substrate is a bearing and theelectroplated tin alloy coating constitutes a bearing overlay.
 13. Aprocess as claimed in claim 1, wherein the electroplating bath comprisesan anionic wetting agent.
 14. A process as claimed in claim 1, whereinthe electroplating bath comprises a salt selected from the groupconsisting of sodium sulphate, ammonium chloride, and combinations ofthe foregoing.